1. Field of the Invention
The present invention relates to refractory submerged entry nozzles which are used in the continuous casting of aluminum killed molten steel. More particularly this invention pertains to an improved sleeve or collar which surrounds the outer portion of the nozzle to protect it from the corrosive effects of molding powder which floats on the surface of the molten metal contained in the continuous casting mold through which the nozzle is partially immersed. This invention also pertains to the refractory composition which is used to make the improved sleeve or collar.
2. Background Art
It is common practice in continuous casting operations to employ a layer of mold powder above the metal surface in the mold in order to capture and prevent the entry of non-metallic inclusions into the molten metal. In addition, the mold powder serves as a lubricant and provides surface protection for the strand of solidified metal as it leaves the mold during a continuous casting procedure. Commonly used mold powders are comprised of a mixture of oxides which have a relatively low melting point and which form a molten slag layer which floats on the surface of the molten metal within the mold. In continuous casting procedures, it is common for the nozzle, through which the molten metal passes, to be slightly submerged in the top layer of the molten metal in the mold. Thus, a portion of the nozzle comes in contact with the slag layer of molding powder which floats on the surface of the molten metal. The area of the nozzle which comes in contact with the floating molding powder is commonly referred to as the slagline or powder line area. This zone of the nozzle which comes in contact with the molding powder undergoes chemical corrosion at a high rate due to the highly corrosive nature of the molding powder. Therefore, in order to protect the portion of the nozzle which comes in contact with the highly corrosive molding powder, it is conventional to provide a sleeve or collar of corrosion resistant material around the portion of the nozzle which comes in contact with the corrosive molding powder. Such nozzles are described in U.S. Pat. Nos. 5,185,300; 5,083,687; 5,348,202; 5,198,126 and 5,046,647, the specifications of which are incorporated herein by reference.
Conventional sleeves or collars are usually confined to the slagline portion of the nozzle. The sleeve is usually confined to this particular portion of the nozzle in order to reduce manufacturing costs. Thus, while it is preferred to confine the sleeve to this narrow zone, this confinement is not essential to the present invention and the sleeve may extend along the entire length or any portion thereof so long as it is present in at least the portion of the nozzle which comes in contact with the molten slag of mold powder which floats on the surface of the molten metal in the mold.
The nozzles to which this invention pertains are sometimes referred to in the art as refractory tubes or shrouds and the like. Therefore, the use of the term xe2x80x9cnozzlexe2x80x9d as used in this invention pertains to nozzles, tubes, shrouds and the like which are used in continuous casting procedures. Thus, the term xe2x80x9cnozzlexe2x80x9d as used herein is inclusive of such conventional refractory tubes, shrouds, and the like which are conventionally used in continuous casting of molten steel. Such nozzles and the use thereof is conventional in the technical field of this invention and are exemplified in U.S. Pat. Nos. 5,046,647; 4,568,007; 5,244,130 and 4,682,718, the specifications of which are incorporated herein by reference.
It is well known in the art that aluminum killed steels tend to form obstructions in conventional alumina-graphite pouring tubes and nozzles. A solution to this problem is described in patent application Ser. No. 08/432,708, the disclosure of which is incorporated herein by reference. In the aforementioned patent application, the nozzle or tube is made from doloma-graphite which prevents unwanted clogging. However, the doloma-graphite material from which the nozzle is fabricated, is easily corroded by the corrosive molding powder discussed above. Nonetheless, such molding powders are considered to be essential since they perform many useful functions in the continuous casting process. In particular, the mold powder acts as insulation which minimizes radiational and convective heat losses and acts as a lubricant to assist the motion of the steel in the mold and acts as an oxidation barrier to prevent ingress of oxygen. In addition, the mold powder functions to cause some metallurgical refining.
The corrosive action causes erosion of the doloma in the doloma-graphite tube or nozzle. Thus, doloma-graphite tubes are also fabricated with sleeves or collars as discussed above.
Such sleeves or collars are conventionally made of materials which resist the corrosive effect of the mold powder. One of the most useful materials for making such sleeves is refractory material containing zirconium oxide. Such sleeves made from zirconium oxide containing refractory material are described in U.S. Pat. Nos. 5,198,126; 5,348,202; 5,083,687 and 5,185,300. Typically these sleeves are made from wear-resistant refractory material which is zirconium oxide/graphite. The zirconium oxide (zirconia)/graphite is conventionally bonded with a binding resin (e.g., phenolic resin) and co-pressed into the outer portion of the nozzle in a narrow zone so as to form the sleeve. The resin is cured thereby converting the resin to resite. The methodology for making conventional zirconia/graphite slagline sleeves on submerged entry nozzles (e.g., nozzles made from alumina-graphite) is disclosed in the article by Sugie et al. entitled xe2x80x9cSubmerged Nozzles for Continuous Casting of Steelxe2x80x9d published in Taikabutsu Overseas, Vol. 1, No. 2, page 78. As noted in this article a zone of zirconium oxide and graphite bonded with phenolic resin is co-pressed into an alumina-graphite tube to produce the sleeve.
When this approach is used with tubes made from doloma-graphite, the forces generated during the conversion of the resin to the resite are expansile in the doloma-graphite body and are contractile in the zirconium oxide-graphite sleeve portion of the body. Since the doloma-graphite body is interior to the zirconium oxide/graphite sleeve portion, the resultant stresses causes fracturing in and around the zirconium oxide-graphite sleeve. Such fracturing is obviously harmful to the structural integrity of the nozzle and is particularly harmful to the structural integrity of the sleeve. Therefore, there is a need in the art to prevent the fracturing which occurs due to the contractile forces generated by the curing resin in the zirconium oxide-graphite sleeve and the expansile forces generated by the curing resin in the doloma-graphite nozzle.
It is an object of this invention to prevent the stress fractures which occur when the resin in the doloma/graphite body of a submerged entry nozzle and the resin of a zirconia/graphite slagline sleeve located around the outer portion of the nozzle is converted from resin to rezite during a curing process which is used to fabricate the nozzle.
It is also an object of this invention to provide a doloma/graphite submerged entry nozzle having a zirconia/graphite slagline sleeve which resists stress fracturing during curing of the resin contained in the doloma/graphite nozzle and the zirconia/graphite sleeve.
It is also an object of this invention to provide resin bonded zirconia/graphite which has reduced contractile tendency when the binding resin is cured and converted from resin to resite.
These and other objects are achieved by adding an effective amount of calcium oxide (CaO) to the zirconium oxide/graphite material which is used to form the sleeve. The calcium oxide may be in the form of dead burned lime or doloma (calcined sintered dolomite). It is theorized that the binding resin used with the zirconium oxide/graphite liberates a small amount of water during the curing procedure and this water escapes thus causing a loss in volume of the resin when the resin is converted to the resite. In contrast, when this same resin liberates a small amount of water in the presence of the doloma contained in the main body of the nozzle, the doloma forms a hydrate which has a larger volume.
The calcium oxide, either in the form of dead burned lime or doloma is included in the resin-zirconium oxide-graphite mixture which is used to form the sleeve. It is theorized that the inclusion of calcium oxide in the resin-zirconium oxide-graphite material is sufficient to counteract the contractile tendency because the water which becomes liberated during the curing process of the resin causes hydration of the added calcium oxide which in turn causes an increase in the volume of the calcium oxide. The hydrated calcium oxide which is formed during the curing of the resin, has a higher volume and thereby counteracts the contractile tendency. Thus, the amount of calcium oxide which is added to the resin-zirconium oxide-graphite material should be sufficient so that the hydrated calcium oxide which is formed during the curing is enough to counteract the contractile tendency associated with the resin cured zirconium oxide-graphite material. Preferably the amount of calcium oxide, either as dead burned lime or doloma, is added in an amount from 0.25% by weight to 10% by weight based upon the amount of zirconium oxide and graphite which is used to form the sleeve.
Typically the zirconium oxide which is used to form the sleeve is stabilized with calcium oxide or magnesium oxide. The calcium oxide or magnesium oxide which is used to stabilize the zirconium oxide becomes part of the crystalline structure of the zirconium oxide. The calcium oxide which is used for the present invention is distinct from the stabilizing calcium oxide which becomes part of the crystalline structure of the zirconium oxide. The calcium oxide which is used in this invention for counteracting the contractile nature of the resin bonded zirconium oxide/graphite material is added as a separate powder and therefore exists as particles which are separate or distinct from the calcium oxide stabilized zirconium oxide.